1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method for utilizing and generating a phase modulated output signal having multiple, independently generated phase delays per cycle of the phase modulated output signal.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives such as the reduction of mercury.
LEDs are semiconductor devices and are driven by direct current. The lumen output intensity (i.e. brightness) of the LED approximately varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the white LEDs or by reducing the average current through duty cycle modulation.
Dimming a light source saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. Many facilities, such as homes and buildings, include light source dimming circuits (referred to herein as “dimmers”).
FIG. 1 depicts a lighting circuit 100 with a conventional dimmer 102 for dimming incandescent light source 104 in response to inputs to variable resistor 106. The dimmer 102, light source 104, and voltage source 108 are connected in series. Voltage source 108 supplies alternating current at mains voltage Vmains. The mains voltage Vmains can vary depending upon geographic location. The mains voltage Vmains is typically 120 VAC (Alternating Current) with a typical frequency of 60 Hz or 230 VAC with a typical frequency of 50 Hz. Instead of diverting energy from the light source 104 into a resistor, dimmer 102 switches the light source 104 off and on many times every second to reduce the total amount of energy provided to light source 104. A user can select the resistance of variable resistor 106 and, thus, adjust the charge time of capacitor 110. A second, fixed resistor 112 provides a minimum resistance when the variable resistor 106 is set to 0 ohms. When capacitor 110 charges to a voltage greater than a trigger voltage of diac 114, the diac 114 conducts and the gate of triac 116 charges. The resulting voltage at the gate of triac 116 and across bias resistor 118 causes the triac 116 to conduct. When the current I passes through zero, the triac 116 becomes nonconductive, i.e. turns ‘off’. When the triac 116 is nonconductive, the dimmer output voltage VDIM is 0 V. When triac 116 conducts, the dimmer output voltage VDIM equals the mains voltage Vmains. The charge time of capacitor 110 required to charge capacitor 110 to a voltage sufficient to trigger diac 114 depends upon the value of current I. The value of current I depends upon the resistance of variable resistor 106 and resistor 112. Thus, adjusting the resistance of variable resistor 106 adjusts the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM is equivalent to adjusting the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM adjusts the average power to light source 104, which adjusts the intensity of light source 104. The term “phase angle” is also commonly referred to as a “phase delay”. Thus, adjusting the phase angle of dimmer output voltage VDIM can also be referred to as adjusting the phase delay of dimmer output signal VDIM. Dimmer 102 only modifies the leading edge of each half cycle of voltage Vmains.
FIG. 2 depicts the periodic dimmer output voltage VDIM waveform of dimmer 102. The dimmer output voltage fluctuates during each period from a positive voltage to a negative voltage. (The positive and negative voltages are characterized with respect to a reference to a direct current (dc) voltage level, such as a neutral or common voltage reference.) The period of each full cycle 202.0 through 202.N is the same as 1/frequency as voltage Vmains, where N is an integer. The dimmer 102 chops the voltage half cycles 204.0 through 204.N and 206.0 through 206.N to alter the duty cycle of each half cycle. The dimmer 102 chops the first half cycle 204.0 (e.g. positive half cycle) at time t1 so that half cycle 204.0 is 0 V from time t0 through time t1 and has a positive voltage from time t1 to time t2. The light source 104 is, thus, turned ‘off’ from times t0 through t1 and turned ‘on’ from times t1 through t2. Dimmer 102 chops the first half cycle 206.0 with the same timing as the second half cycle 204.0 (e.g. negative half cycle). So, the duty cycles of each half cycle of cycle 202.0 are the same. Thus, the full duty cycle of dimmer 102 for cycle 202.0 is represented by Equation [1]:
                              Duty          ⁢                                          ⁢          Cycle                =                                            (                                                t                  2                                -                                  t                  1                                            )                                      (                                                t                  2                                -                                  t                  0                                            )                                .                                    [        1        ]            
When the resistance of variable resistance 106 is increased, the duty cycle of dimmer 102 decreases. Between time t2 and time t3, the resistance of variable resistance 106 is increased, and, thus, dimmer 102 chops the full cycle 202.N at later times in the first half cycle 204.N and the second half cycle 206.N of the full cycle 202.N with respect to cycle 202.0. Dimmer 102 continues to chop the first half cycle 204.N with the same timing as the second half cycle 206.N. So, the duty cycles of each half cycle of cycle 202.N are the same. Thus, the full duty cycle of dimmer 102 for cycle 202.N is:
                              Duty          ⁢                                          ⁢          Cycle                =                                            (                                                t                  5                                -                                  t                  4                                            )                                      (                                                t                  5                                -                                  t                  3                                            )                                .                                    [        2        ]            
Since times (t5−t4)<(t2−t1), less average power is delivered to light source 104 by the sine wave 202.N of dimmer voltage VDIM, and the intensity of light source 104 decreases at time t3 relative to the intensity at time t2.
The voltage and current fluctuations of conventional dimmer circuits, such as dimmer 102, can destroy LEDs. U.S. Pat. No. 7,102,902, filed Feb. 17, 2005, inventors Emery Brown and Lodhie Pervaiz, and entitled “Dimmer Circuit for LED” (referred to here as the “Brown patent”) describes a circuit that supplies a specialized load to a conventional AC dimmer which, in turn, controls a LED device. The Brown patent describes dimming the LED by adjusting the duty cycle of the voltage and current provided to the load and providing a minimum load to the dimmer to allow dimmer current to go to zero.
Exemplary modification of leading edges and trailing edges of dimmer signals is discussed in “Real-Time Illumination Stability Systems for Trailing-Edge (Reverse Phase Control) Dimmers” by Don Hausman, Lutron Electronics Co., Inc. of Coopersburg, Pa., U.S.A., Technical White Paper, December 2004 (“Hausman Article), and in U.S. Patent Application Publication, 2005/0275354, entitled “Apparatus and Methods for Regulating Delivery of Electrical Energy”, filed Jun. 10, 2004, inventors Hausman, et al. (“Hausman Publication”) Both the Hausman Article and Hausman Publication are incorporated herein by reference in their entireties.
Thus, conventional dimmers provide dependently generated phase delays per cycle of a phase modulated signal.